This invention relates to a system used for measuring and monitoring temperature using ultrasonic thermometry, and more particularly to a system that measures temperature using solid rods of material that function as the temperature sensor.
Currently, thermocouples are used extensively in the monitoring of industrial process temperatures. Thermocouples are a pair of bare wires, each of which may be in the range of 0.005xe2x80x3 to 0.030xe2x80x3 in diameter, made from two dissimilar metals, joined at one end to form a temperature sensitive electro-voltaic junction. The junction at the tip of the wires is the part of the thermocouple that is used to perform the temperature measurement. The thermocouple assembly, generally constructed as a long, rod-like assembly, is typically from xe2x85x9 inch to 1 inch in overall diameter, with a functional length of from 1 or 2 inches, to several feet. It is partially inserted into the process, the temperature of which is being measured, with the temperature-sensitive junction at the tip being placed in the area of interest in the process. Access to the process is typically obtained via a port in the furnace or reactor in which the process is occurring. The two open ends of the wires at the other end produce a voltage curve that is a function of temperature. The voltage curve produced for a given range of temperatures is primarily a function of the types of dissimilar metals, and the accuracy or calibration of it is primarily a function of the purity of the alloys used to make the thermocouple.
The most widely used thermocouples for measuring high temperatures are platinum/rhodium alloy combinations, theoretically useful for measuring temperatures up to 1,700 C. in oxidizing or inert environments, and tungsten/rhenium alloy combinations, theoretically useful for measuring temperatures up to 2,300 C. in vacuum, inert, or reducing environments. The bare thermocouple wire pairs are usually separated along their length by insulation packing or ceramic beads that are strung along the length of the wires. The thermocouples are often sheathed in a protection tube, and the protection tube is sometimes purged with an inert gas to increase thermocouple lifetime. Both thermocouple types have relatively short lifetimes. They also have increasing end-of-life inaccuracies due to inadvertent oxidation, reduction, or other chemical adulterations to the relatively fine wire alloys or electro-voltaic junction from temperature and process exposure. Additional inaccuracies are introduced as the ceramic insulation becomes increasingly conductive at higher temperatures, thus shunting the small voltage produced by the thermocouple. Thermocouples additionally are susceptible to electromagnetic interference often found in industrial environments, which adds uncertainty and error to such measurements.
As mentioned previously, thermocouple wires are typically in the range of 0.005xe2x80x3 to 0.030xe2x80x3 in diameter. Because of their small physical diameter, they are readily susceptible to chemical attack and performance degradation. Thermocouple wires can be made larger, however the cost at $5 to $35 per inch, depending on type and wire gauge, is prohibitive for industry-standard high temperature platinum/rhodium or tungsten/rhenium wire pairs greater than the mentioned diameter range.
Ultrasonic thermometry can substantially improve the shortcomings of the industry standard thermocouple. A major improvement is to lifetime and stability. This is accomplished by using a solid rod of material to measure the temperature rather than two thin dissimilar metal wires. This bare sensing rod is much more substantive than bare thermocouple wires, being approximately in the range of 0.100xe2x80x3 to 0.250xe2x80x3 in diameter, and is similar in functional length to the thermocouple. The ultrasonic thermometer rod is directly substituted into the same process as the thermocouple that it physically replaces. The larger physical size of the bare temperature-sensing rod material, along with selecting application compatible rod materials or materials resistant to very high temperatures, provide a much greater lifetime than with thermocouples. Additionally, the lower cost of ultrasonic thermometer rod material, in the range of $1 to $4 per inch, is much more favorable than that of the high temperature thermocouple wire that it replaces.
Ultrasonic thermometry relies on the fact that the speed of sound in a solid is a function of temperature. The propagation velocity of ultrasound in a solid material is a function of both the density and the modulus of elasticity of the material, both of which are functions of temperature. In a long, solid rod of material, the mathematical relationship can be expressed as Ve=SQRT(E(t)/P(t)), where Ve is the extensional wave velocity in a long rod, SQRT is square root, E(t) is Young""s modulus as a function of temperature, and P(t) is density as a function of temperature. This physical phenomenon is the basis of ultrasonic thermometry.
Ultrasonic thermometer systems exist that measure temperature with solid rods of material that function as the temperature sensor. Such systems have used both pulse-echo or continuous wave techniques to measure temperature. Examples of pulse-echo systems and temperature sensing probes are shown and described in U.S. Pat. No. 4,772,131 issued to Varela et al, U.S. Pat. No. 4,483,630 issued to Varela, U.S. Pat. No. 3,597,316 issued to Lynnworth, U.S. Pat. No. 3,540,265 issued to Lynnworth, U.S. Pat. No. 3,633,424 issued to Lynnworth, and U.S. Pat. No. 3,717,033 issued to Gordon et al. These ultrasonic thermometer systems and probes generally function by coupling a short ultrasonic pulse into the probe rod with a transducer. Along the length of the rod, circumferential grooves are cut which reflect some of the ultrasonic energy back to the transducer, thus creating an echo signal. These systems rely upon accurately measuring the time between two return pulses as the representative measure of temperature. Two such reflected or echo signals from two adjacent grooves, or a signal from one groove and a signal from the end of the rod, establish a temperature zone. As the temperature of the zone changes, the transition time of the ultrasonic pulse through the zone also changes, thus providing a measurable indication of average temperature and changes in average temperature of the temperature zone defined by the reflections. A zone may be from less than an inch to many inches in length.
The time position of the return pulses in prior art has been established (a) by comparing the pulses to a threshold that is above the noise level, thus triggering a comparator, (b) by detecting a zero crossing of the reflected signals after crossing a threshold, or (c) by some combination of these techniques. In all cases, these systems attempt to establish the accurate time position of the peaks in the energy of the reflected pulses relative to a clock, but only do so indirectly by either setting a threshold somewhere below the actual reflected energy peak, or by detecting an event like a zero crossing that occurs before or after the actual peak. These systems have inherent inaccuracies in that the peak widths or slopes may change due to external factors not related to the temperature being measured, thereby causing an apparent change in measured temperature, which is actually an error. Such inaccuracies can be caused by environmental thermal effects on the exciting transducer, shifts in the superposition of the composite multi-axis components of the transducer response relative to each other, or electromagnetic noise.
In the known systems described above, no mechanism is provided for conveniently delivering the temperature calibration with the probe when it is installed or replaced by the user of such a system. Nor is any provision made for temperature compensating the probe response due to changes in ambient environmental temperature changes.
Further compromising the performance of prior art ultrasonic thermometers has been the complexity of the reflected waveforms, especially in pulse-echo systems. In pulse-echo systems, a single short pulse, perhaps 0.1 to 10 microseconds wide, is sent down the length of the sensing rod. This ultrasonic energy then reflects off the notches and the end of the rod, creating the echo reflections that are then processed to extract the timing information necessary to establish the temperature of the zone. The accuracy of such measurements is severely altered by a number of unavoidable effects that have plagued prior art systems including: 1) there is a limited ability to create a perfectly sinusoidal or perfectly square pulse with which to drive the transducer because of parasitic capacitances and inductances, 2) natural ringing in the acoustic system can only be damped to a limited degree, 3) mode conversion of the pulse as it travels down the length of rod converts from a primarily extensional wave at creation at the transducer to a multi-modal waveform by the time it makes a xe2x80x9cround tripxe2x80x9d down the rod and back to the transducer, and 4) dispersion of the square pulse as different frequencies and harmonics in the initial pulse travel at slightly different velocities thereby degrading the xe2x80x9csquarenessxe2x80x9d of the initial pulse. These effects all combine to make the signal processing of the reflected waveforms a complex one. Simple techniques used by prior art systems, such as detecting zero crossings and crossing fixed thresholds, cannot hope to properly extract the precise timing information that is required given the complexity of the reflected waveforms.
U.S. Pat. No. 4,215,582 issued to Akita is an example of a continuous wave ultrasonic thermometer system that measures temperature by comparing the phase difference caused by temperature changes in two continuously received signals. Although it uses a different technique from the present invention, no provision is disclosed or suggested for the calibration of the temperature sensors or for conveniently delivering the calibration with the probe when it is installed or replaced by the user of such a system.
The rod materials used in the present invention are selected to be have high stability of their grain structures at their operating temperatures and environments for extended periods. In prior art systems, such as the device described in U.S. Pat. No. 3,350,942, high temperature materials are listed and acoustic properties are referenced, however the stability of grain structure over time and temperature are not considered. Change in grain structure through grain growth will cause a permanent change in the ultrasound velocity within the sensing rod and thus cause an apparent shift in measured temperature.
There have been past attempts to commercialize some of the ultrasonic thermometry systems described above, but none have been successful. Primitive signal processing that cannot properly process the complex signals involved in making an ultrasonic thermometer measurement, or is difficult to maintain adjustment have been major difficulties to overcome. Difficulty in installing probe calibration information, and the inability to integrate easily with an existing thermocouple monitoring infrastructure have made trials of an unproven ultrasonic temperature measurement technology difficult to evaluate by potential commercial customers. Thermal or electromagnetic environmental susceptibilities and unreliable operation have resulted from poorly designed signal processing circuitry. Unrepeatable results and temperature shifts with repeated use have resulted from improperly selected or prepared materials used as the sensing rod elements. High costs have resulted due to overly complex electronic hardware systems making the products less attractive to potential buyers. These are some of the key factors that have contributed to the lack of commercial success with prior art systems described above.
It is therefore a principal object of the present invention to provide an ultrasonic thermometer system that produces repeatable, reliable and accurate high temperature measurements over long periods in severe chemical environments found in industrial processes using more sophisticated signal processing.
Another object of the present invention is to provide an ultrasonic thermometer system that compensates its temperature measurements for changes in the ambient temperature of the transducer within the temperature measurement probe, and that also compensates for probe-to-probe variations in signal amplitude that arise from small differences in the transducer material, construction techniques, or other manufacturing variations.
Another object of the present invention is to provide an ultrasonic thermometer system that uses high temperature, grain-stable materials in the sensing element with lifetimes that exceed the lifetimes of the thermocouple sensing elements that they replace by a factor of 3 to 10.
A still further object of the present invention is to provide an ultrasonic thermometer system that easily integrates into existing temperature monitoring infrastructures by providing ultrasonic thermometer probes that are easily replaced without the user worrying about calibration, and that may provide as one of its outputs, a thermocouple-compatible output that will attach to a variety of temperature monitoring systems that use a thermocouple as an input.
The ultrasonic thermometer system of the present invention is generally a rod or probe of high temperature, grain-stabilized material that has a magnetostrictive or piezoelectric transducer bonded to one end. The transducer is excited by a transducer driver, creating short, periodic, ultrasonic pulses that travel down the length of the rod in a xe2x80x9cpulse-echoxe2x80x9d fashion. Along the length of the rod, circumferential grooves (notches) are cut which reflect some of the ultrasonic energy back to the transducer thus creating a reflected or echo signal. Two such reflected signals from two adjacent grooves, or a signal from one groove and a signal from the end of the rod, establish a temperature zone. This is the zone of interest to the user, which would be inserted into the user""s process that needs to have the temperature monitored. As the temperature of the zone changes, the transition time of the ultrasonic pulse through the zone also changes, thus providing a measurable indication of average temperature and changes in average temperature of the temperature zone. There may be one or multiple temperature zones on one rod.
The probe head electronics amplifies, filters, and performs other signal conditioning functions on the signal of the reflected pulse. This reflected signal is then digitized, keeping accurate track of positioning in time. A digital processor then performs further filtering, signal conditioning, noise reduction, and analytical functions on the digitized signal. These functions include, among other things, short and long term signal averaging, level and temporal thresholding, and correlation and filtering. During the correlation processing, the digitized reflected signal is compared with a correlation template that resembles the shape of the expected reflected signal. The processor correlates the template to the reflected temperature zone boundary signals to precisely establish their position in time and changes in their position in time. There can be one or more echo templates used, i.e. each notch may have its own template or a set of notches may share a common template. The use of correlation techniques to determine notch echo timing provides a great improvement over prior art systems in noise reduction and immunity from other external environmental effects on the echo signals. It also takes into account and corrects for echo pulse decomposition due to propagation mode conversion and signal dispersion, conditions that prior art systems were unable to correct. The calibration information contained in the replaceable probe assembly is read and stored by the processor, providing the necessary information for the processor to determine the average temperature of the temperature zone or zones, given the dimensions of the zone or zones, the material from which the probe is made, and the signal template itself.
The processor, using a look-up table, interpolation between calibration points, curve fit equation of the probe material, or other means commonly used in the art, converts the temperature zone timing information into absolute temperature readings and sends them in digital form to the probe head""s data connector. This digital information may optionally be sent to a transmitter, a computer, or any other digital control or display device. The transmitter would, for example, convert the digital representation of temperature to a visual display, and/or into an analog output that is linear, or that follows the calibration curve of the popular platinum-rhodium, tungsten-rhenium, or other thermocouple types. The digital output from the head can also be converted into other forms of analog or digital output, such as 4-20 mA, 0-10V, or the like. The digital output from the probe head electronics may also be used directly without further conversion.
These and other features and objects of the present invention will be more fully understood from the following detailed description which should be read in light of the accompanying drawings in which corresponding reference numerals refer to corresponding parts throughout the several views.